1. Technical Field of the Invention
The present invention relates generally to high-frequency photo-amplifier circuits, and more particularly to power supply rejection of high-frequency photo-amplifier circuits.
2. Description of Related Art
The ability of a circuit to disregard noise on the supply line is referred to as the power supply rejection of the circuit. High frequency circuits have traditionally had poor power supply rejection due to the high bandwidth handled by the circuits. For example, in high frequency photo-amplifier circuits with large bandwidths in the gigabit/s range, it has become increasingly difficult to filter noise on the supply line.
In addition, with the increase in the number of components placed on a circuit board, there has been a corresponding increase in the amount of broadband noise observed on the supply line. If the level of noise is significant enough, the integrity of any signal produced by the photo-amplifier circuit may be degraded.
Conventional mechanisms for improving the power supply rejection in photo-amplifier circuits have included complex and elaborate filters and/or differential amplifier circuits, used for common mode rejection of noise signals. However, as bandwidths increase and the number of components per board increase, there is a need for improved power supply rejection in photo-amplifier circuits.
Embodiments in accordance with the invention provide a symmetrical photo-amplifier circuit with improved power supply rejection. Typically, the photo-amplifier circuit includes a signal circuit and a reference circuit, in which the inductances and parasitic capacitances from mechanical sources, such as bond wires and substrates, are matched between the signal circuit and the reference circuit. The signal circuit and reference circuit are typically coupled to a filly differential preamplifier circuit for common mode rejection of noise signals on the power supply line. The signal circuit typically receives an optical signal on the power supply line and is coupled to an input of a first amplifier. The reference circuit is typically electrically similar to the signal circuit and receives a noise signal substantially identical to the noise signal received at the signal circuit. The reference circuit is further coupled to an input of a second amplifier. Signals output from the first and second amplifiers are input to a differential amplifier for common mode rejection of the noise signals.